Online works for any subject involving a lot of reading / visual aids, but not complex formulas or lab. Math may be better done using a textbook, at least in the coming grades, and of course science is going to be more fun if it's focused on experiments rather than just theory. Writing may or may not work depending on if the online school you're using offers grading.

Short version - you can use an online school for certain subjects to save time, but I would personally avoid using it for everything. Some things are just better done offline.